Semiconductor device

ABSTRACT

A semiconductor device includes a semiconductor substrate having a principal surface, and an insulating film formed on the principal surface and continuously covering a top surface of a first boundary region and a top surface of a second boundary region, the first boundary region including a boundary between a well layer and a RESURF layer, the second boundary region including a boundary between the RESURF layer and a first impurity region. The semiconductor device further includes a plurality of lower field plates formed in the insulating film in such a manner that the plurality of lower field plates do not lie directly above the first and second boundary regions, and a plurality of upper field plates formed on the insulating film in such a manner that the plurality of upper field plates do not lie directly above the first and second boundary regions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device used, e.g., forpower conversion, power control, etc.

2. Background Art

Japanese Laid-Open Patent Publication No. 2010-245281 discloses asemiconductor device having a field plate structure and a RESURF layerformed therein. The field plate structure and the RESURF layer areformed in the periphery of the semiconductor device in order to increasethe breakdown voltage of the device.

It is desirable to minimize the size of semiconductor devices whilemaintaining the breakdown voltage at a high enough level. Therefore,there is a need to provide a semiconductor device which is smaller thanthe semiconductor device disclosed in the above-mentioned patentpublication and which yet has an adequately high breakdown voltage.

SUMMARY OF THE INVENTION

The present invention has been made to solve this problem. It is,therefore, an object of the present invention to provide a semiconductordevice of reduced size, yet having an adequately high breakdown voltage.

According to one aspect of the present invention, a semiconductor deviceincludes a semiconductor substrate having a principal surface, a firstimpurity region of a first conductivity type formed in the semiconductorsubstrate, a RESURF layer of a second conductivity type formed in thesemiconductor substrate along the principal surface, a well layer of thesecond conductivity type formed in the semiconductor substrate adjacentthe RESURF layer and along the principal surface, a channel stopper ofthe first conductivity type formed in the semiconductor substrateadjacent the RESURF layer and along the principal surface, with thefirst impurity region interposed between the channel stopper and theRESURF layer, an insulating film formed on the principal surface andcontinuously covering a top surface of a first boundary region and a topsurface of a second boundary region, the first boundary region includinga boundary between the well layer and the RESURF layer, the secondboundary region including a boundary between the RESURF layer and thefirst impurity region, a plurality of lower field plates formed in theinsulating film in such a manner that the plurality of lower fieldplates do not lie directly above the first and second boundary regions,and a plurality of upper fieldplates formed on the insulating film insuch a manner that the plurality of upper field plates do not liedirectly above the first and second boundary regions.

According to another aspect of the present invention, a semiconductordevice includes a semiconductor substrate having a principal surface, afirst impurity region of a first conductivity type formed in thesemiconductor substrate, a well layer of a second conductivity typeformed in the semiconductor substrate along the principal surface, achannel stopper of the first conductivity type formed in thesemiconductor substrate away from the well layer and along the principalsurface, and a RESURF layer formed in the semiconductor substratebetween the well layer and the channel stopper and along the principalsurface, and having an impurity concentration of the second conductivitytype which gradually decreases away from the well layer toward thechannel stopper.

According to another aspect of the present invention, a semiconductordevice includes a semiconductor substrate having a principal surface, afirst impurity region of a first conductivity type formed in thesemiconductor substrate, a RESURF layer of a second conductivity typeformed in the semiconductor substrate and along the principal surface, awell layer of the second conductivity type formed in the semiconductorsubstrate adjacent the RESURF layer and along the principal surface, aconcentration gradient reducing section formed in the portion of thewell layer adjacent the RESURF layer in such a manner that the impurityconcentration gradient of the second conductivity type between the welllayer and the RESURF layer is reduced, and a gate wire formed directlyabove the concentration gradient reducing section.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a semiconductor device in accordance with afirst embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1, andprimarily shows the electric field reducing region;

FIG. 3 is a diagram showing the electric field intensity across thesurface of the RESURF layer and the adjoining surfaces which extendalong the principal surface;

FIG. 4 is a cross-sectional view of a variation of the semiconductordevice of the first embodiment;

FIG. 5 is a diagram showing the capacitances in the semiconductor deviceshown in FIG. 4;

FIG. 6 is a cross-sectional view of a semiconductor device in accordancewith a second embodiment of the present invention;

FIG. 7 includes cross-sectional views showing the way in which thep-well layer and the RESURF layer are formed in the same process;

FIG. 8 is a cross-sectional view of a semiconductor device in accordancewith a third embodiment of the present invention;

FIG. 9 includes cross-sectional views showing the method of forming theRESURF layer in accordance with the third embodiment;

FIG. 10 is a cross-sectional view of a semiconductor device inaccordance with a fourth embodiment of the present invention;

FIG. 11 includes cross-sectional views showing the method of forming theRESURF layer in accordance with the fourth embodiment of the presentinvention;

FIG. 12 is a cross-sectional view of a semiconductor device inaccordance with a fifth embodiment of the present invention;

FIG. 13 is a cross-sectional view of a semiconductor device inaccordance with a sixth embodiment of the present invention;

FIG. 14 is a cross-sectional view of a semiconductor device having theRESURF layer shown in FIG. 10 described in connection with the fourthembodiment;

FIG. 15 is a cross-sectional view of a semiconductor device having theRESURF layer shown in FIG. 12 described in connection with the fifthembodiment;

FIG. 16 is a cross-sectional view of a semiconductor device inaccordance with a seventh embodiment of the present invention;

FIG. 17 is a cross-sectional view of a semiconductor device having theRESURF layer shown in FIG. 10 described in connection with the fourthembodiment;

FIG. 18 is a cross-sectional view of a semiconductor device having theRESURF layer shown in FIG. 12 described in connection with the fifthembodiment;

FIG. 19 is a cross-sectional view of a semiconductor device inaccordance with an eighth embodiment of the present invention; and

FIG. 20 is a cross-sectional view of a variation of the semiconductordevice of the eighth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe accompanying drawings. It should be noted that the semiconductordevice of each of these embodiments will be specifically described inconnection with an IGBT (insulated gate bipolar transistor).

First Embodiment

FIG. 1 is a plan view of a semiconductor device in accordance with afirst embodiment of the present invention. The semiconductor device 10has a device forming region provided in the central portion of its chip,and an emitter electrode 12 and a gate electrode pad 14 are formed onthe surface of the device forming region. An electric field reducingregion is provided so as to surround the device forming region, that is,provided along the periphery of the chip, and the surface of theelectric field reducing region is covered by a passivation film 16.

FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1, andprimarily shows the electric field reducing region. The semiconductordevice 10 includes a semiconductor substrate 20. The semiconductorsubstrate 20 is formed of Si. An n-type first impurity region 21(hereinafter referred to as the n-type region 21) is formed in thesemiconductor substrate 20. Further, a p-type RESURF (or reduced surfacefield) layer 24 is formed in the semiconductor substrate 20 along itsprincipal surface 20 a. The impurity concentration and the depth (orthickness) of the RESURF layer 24 are such that the RESURF layer 24satisfies its full depletion condition (or RESURF condition). A p-typewell layer 22 is formed in the semiconductor substrate 20 adjacent theRESURF layer 24 and along the principal surface 20 a. Further, a p-typebase layer (or p-base layer) 29 is formed adjacent the side of thep-type well layer 22 opposite the side facing the RESURF layer 24. Thep-well layer 22 has a depth equal to or greater than the depth of thep-base layer 29, and has a function to reduce the electric fieldintensity at the periphery of the p-base layer 29. Further, the p-baselayer 29 has a plurality of MOS (metal oxide semiconductor) structuresformed therein so that the p-base layer 29 serves as a region in whichchannels are formed. A boundary region including the boundary betweenthe p-well layer 22 and the RESURF layer 24 is hereinafter referred toas the first boundary region 23. While not shown, it should be notedthat trenches are formed to extend, e.g., from the surface of thesemiconductor substrate to the n-type region, and each MOS structuredescribed above has the following: a gate electrode formed by fillingone of the trenches with polysilicon, with a gate oxide film interposedbetween the polysilicon and the sidewalls of the trench; and ann⁺-emitter layer formed along the sides of the trench and extending apredetermined depth from the substrate surface. Further, the on-offoperation of the IGBT is effected by controlling the channels formed bythe voltage applied to these gate electrodes.

An n-type channel stopper 26 is formed in the semiconductor substrate 20along the principal surface 20 a. The channel stopper 26 is formed inthe edge portions (or periphery) of the semiconductor device 10; thatis, the channel stopper 26 is disposed away from the p-well layer 22 andadjacent the RESURF layer 24, with the n-type region 21 between thechannel stopper 26 and the RESURF layer 24. A boundary region includinga boundary between the RESURF layer 24 and the n-type region 21 ishereinafter referred to as the second boundary region 25. An insulatingfilm 30 is formed on the principal surface 20 a and continuously coversthe surface of the first boundary region 23 and the surface of thesecond boundary region 25. The insulating film 30 is a silicon oxidefilm or the like deposited, e.g., by CVD.

A plurality of lower field plates 32 are formed in the insulating film30. These lower field plates 32 include lower field plates 32 a, 32 b,32 c, and 32 d. The lower field plates 32 a, 32 b, 32 c, and 32 d areformed in such a manner that they do not lie directly above the firstboundary region 23 and the second boundary region 25. It should be notedthat the plurality of lower field plates 32 are concentric and annularas viewed in plan, and in accordance with the present invention thematerial of these lower field plates 32 is doped polysilicon.

A plurality of upper field plates 34 are formed on the insulating film30. These upper field plates 34 include upper field plates 34 a, 34 b,and 34 c. The upper field plates 34 a, 34 b, and 34 c are formed in sucha manner that they do not lie directly above the first boundary region23 and the second boundary region 25. The plurality of upper fieldplates 34, like the lower field plates 32, are concentric and annular asviewed in plan.

An emitter electrode 12 is formed on the p-well layer 22. The emitterelectrode 12 is formed in contact with the p-base layer 29 and thep-well layer 22 and extends over the insulating film 30 directly abovethe first boundary region 23. That is, the emitter electrode 12 isformed to cover the insulating film 30 directly above the first boundaryregion 23.

A channel stopper electrode 36 is formed on the channel stopper 26. Thechannel stopper electrode 36 is formed in contact with the channelstopper 26 and extends over the insulating film 30 directly above thesecond boundary region 25. That is, the channel stopper electrode 36 isformed to cover the insulating film 30 directly above the secondboundary region 25. It should be noted that the upper field plates 34,the emitter electrode 12, and the channel stopper electrode 36 areformed, e.g., of a metal film of aluminum, etc. The upper fieldplates34, the emitter electrode 12, and the channel stopper electrode 36 whichoverlie the insulating film 30 partially overlap the lower field plates32 in the insulating film 30 with the insulating film 30 therebetween,thereby forming the desired capacitances.

A passivation film 16 is formed to cover the electric field reducingregion. An n-type buffer layer 38 is formed on the opposite side of thesemiconductor substrate 20 from the principal surface 20 a and is incontact with the n-type region 21. A p-type collector layer 40 is formedin contact with the buffer layer 38. A collector electrode 42 made of ametal film, etc. is formed in contact with the collector layer 40. Thesemiconductor device 10 of the first embodiment is provided with thecomponents described above.

It should be noted that in order to increase the breakdown voltage ofthe semiconductor device, it is desirable to equalize the electric fieldintensity across the surface of the RESURF layer and the adjoiningsurfaces extending along the principal surface of the substrate. It hasbeen found in some cases, however, that the electric field intensity atthe first and second boundary regions is too high to increase thebreakdown voltage of the semiconductor device. The electric fieldintensity produced across the surface of the RESURF layer and theadjoining surfaces will be described with reference to FIG. 3. FIG. 3 isa diagram showing the electric field intensity across the surface of theRESURF layer 24 and the adjoining surfaces which extend along theprincipal surface 20 a. Specifically, in FIG. 3, the dashed lineindicates the electric field intensity distribution across the RESURFlayer and the adjoining surfaces in a conventional semiconductor devicein which the lower field plates are disposed in the insulating filmdirectly above the first and second boundary regions. The solid lineindicates the electric field intensity distribution across the RESURFlayer and the adjoining surfaces in the semiconductor device 10 of thefirst embodiment.

In the case where the lower field plates are disposed in the insulatingfilm directly above the first and second boundary regions, the electricfield intensity is relatively high around and near the edges of thelower field plate closest to the first boundary region (this lower fieldplate being hereinafter referred to as the first lower field plate) andaround and near the edges of the lower field plate closest to the secondboundary region (this lower field plate being hereinafter referred to asthe second lower field plate). As a result, the electric fieldintensities at particular portions around the first and second boundaryregions are significantly high, and the electric field intensity acrossthe surface of the RESURF layer and the adjoining surfaces is notuniform, as indicated by the dashed line in FIG. 3. In order to overcomethis problem, the thickness of the insulating film may be increased toincrease the distance between the first lower field plate and the firstboundary region and the distance between the second lower field plateand the second boundary region. However, an increase in the thickness ofthe insulating film results in an increase in the height of steps on thesemiconductor substrate which are associated with the insulating film,making it difficult to manufacture the semiconductor device, resultingin increased cost of the deposition of the films of the device.

On the other hand, the configuration of the semiconductor device of thefirst embodiment makes it possible to prevent any particular portionfrom being subjected to a high electric field intensity, as well as tosubstantially equalize the electric field intensity across the surfaceof the RESURF layer and the adjoining surfaces. Specifically, in thesemiconductor device 10 of the first embodiment, the plurality of lowerfield plates 32 are not formed directly above the first boundary region23 and the second boundary region 25. This means that the first lowerfield plate 32 a can be spaced an adequate distance from the firstboundary region 23 and the second lower field plate 32 d can be spacedan adequate distance from the second boundary region 25 so as to reducethe electric field intensities around and near the edges of the firstlower field plate 32 a and the second lower field plate 32 d. Therefore,it is possible to reduce the electric field intensity at the first andsecond boundary regions and thereby increase the breakdown voltage ofthe semiconductor device.

Further, in the semiconductor device 10 of the first embodiment, theemitter electrode 12 is formed to extend over the insulating film 30directly above the first boundary region 23 and overlaps a portion ofthe lower field plate 32 a. This shifts the peak of electric fieldintensity on the first boundary region side of the electric fieldreducing region toward the center of the RESURF layer 24 while reducingthe electric field intensity. Further, the channel stopper electrode 36is formed to extend over the insulating film 30 directly above thesecond boundary region 25 and overlaps a portion of the lower fieldplate 32 d. This shifts the peak of electric field intensity on thesecond boundary region side of the electric field reducing region towardthe center of the RESURF layer 24 while reducing the electric fieldintensity. Thus the electric field reducing region of the firstembodiment has high breakdown voltage-enhancing effect, making itpossible to manufacture a reduced size semiconductor device having anadequately high breakdown voltage.

FIG. 4 is a cross-sectional view of a variation of the semiconductordevice of the first embodiment. This semiconductor device includes aplurality of lower field plates 50, which include lower field plates 50a, 50 b, 50 c, and 50 d. This semiconductor device is characterized bythe locations of the first lower field plate 50 a and the second lowerfield plate 50 d. The first capacitance formed between the first lowerfield plate 50 a and the emitter electrode 12 and the second capacitanceformed between the second lower field plate 50 d and the channel stopperelectrode 36 are greater than the third capacitance formed between oneof the plurality of lower field plates 50 and one of the plurality ofupper field plates 34. This magnitude relationship between thecapacitances results from the fact that the overlap width a between thefirst lower field plate 50 a and the emitter electrode 12 and theoverlap width c between the second lower field plate 50 d and thechannel stopper electrode 36 are greater than the overlap width bbetween one of the plurality of upper field plates 34 and one of theplurality of lower field plates 50. FIG. 5 is a diagram showing thecapacitances in the semiconductor device shown in FIG. 4. As a result ofthe overlap width relationship described above, the capacitance C1between the first lower field plate 50 a and the emitter electrode 12and the capacitance C8 between the second lower field plate 50 d and thechannel stopper electrode 36 are greater than a capacitance C2, C3, C4,C5, C6, or C7.

In the configuration of this variation, since the capacitances C1 and C8are large, the potentials of the first lower field plate 50 a and thesecond lower field plate 50 d can be reduced to increase the advantagesof the semiconductor device 10 described above.

In addition to the above alteration, various other alterations may bemade to the semiconductor device of the first embodiment. For example,the present invention does not necessarily require that the emitterelectrode 12 be provided directly above the first boundary region 23 andthe channel stopper electrode 36 be provided directly above the secondboundary region 25.

Second Embodiment

FIG. 6 is a cross-sectional view of a semiconductor device in accordancewith a second embodiment of the present invention. The semiconductordevice of the second embodiment has many features common to thesemiconductor device of the first embodiment. Therefore, the followingdescription of the semiconductor device of the second embodiment will belimited to the differences from the semiconductor device of the firstembodiment.

The RESURF layer 52 of the second embodiment is formed by first forminga plurality of p-type regions in the principal surface 20 a of thesemiconductor substrate 20 and then heat-treating the substrate. As aresult of this heat treatment, the plurality of p-type regions togetherform a single continuous p-type region. The RESURF layer 52 satisfiesits full depletion condition (or RESURF condition). The RESURF layer 52is advantageous in that, being made up of a plurality of p-type regions,it can be formed along with the p-well layer 22 in the same process.This feature is described below.

FIG. 7 includes cross-sectional views showing the way in which thep-well layer and the RESURF layer are formed in the same process.Specifically, FIG. 7A shows the electric field reducing regionimmediately after an ion implantation step, and FIG. 7B shows theelectric field reducing region after the ion-implanted impurities havebeen diffused by subjecting the substrate to heat treatment. In thisforming process, first an insulating film 53 a is formed on the n-typeregion 21 by a general semiconductor manufacturing technique, and aresist 53 b is formed on the insulating film 53 a by a photolithographictechnique, etc. The resist 53 b has formed therein openings havingwidths W1 to W13. These widths W1 to W13 are equal. Ions are thenimplanted using the resist 53 b as a mask, thereby forming a pluralityof p-type regions in the principal surface 20 a of the semiconductorsubstrate 20. FIG. 7A shows the p-type regions immediately after theyhave been formed. The resist 53 b is then removed, and heat treatment isperformed to diffuse the impurities in the plurality of p-type regionsand thereby simultaneously form the p-well layer 22 and the RESURF layer52, as shown in FIG. 7B. The use of this forming method makes itpossible to manufacture a reduced size semiconductor device having anadequately high breakdown voltage at low cost.

Third Embodiment

FIG. 8 is a cross-sectional view of a semiconductor device in accordancewith a third embodiment of the present invention. The semiconductordevice of the third embodiment has many features common to thesemiconductor device of the first embodiment. Therefore, the followingdescription of the semiconductor device of the third embodiment will belimited to the differences from the semiconductor device of the firstembodiment.

The RESURF layer 60 of the third embodiment is formed by first forming aplurality of p-type regions in the principal surface 20 a of thesemiconductor substrate 20 in such a manner that they are closely spacedon the p-well layer 22 side of the electric field reducing region andwidely spaced on the channel stopper 26 side of the electric fieldreducing region, and then heat-treating the substrate. After the heattreatment, the impurity concentration in the RESURF layer 60 is high onthe p-well layer 22 side, on which the p-type regions are closelyspaced, and low on the channel stopper 26 side, on which the p-typeregions are widely spaced. The RESURF layer 60 is formed so as tosatisfy its full depletion condition (or RESURF condition).

An insulating film 62 is formed on the principal surface 20 a and coversthe first boundary region 23 and the second boundary region 25. Aplurality of lower field plates 64 are formed in the insulating film 62.These lower field plates 64 include lower field plates 64 a, 64 b, 64 c,and 64 d. The lower field plate 64 a is connected to the emitterelectrode 12. The lower field plate 64 d is connected to the channelstopper electrode 36.

A method of forming the RESURF layer 60 will now be specificallydescribed. FIG. 9 includes cross-sectional views showing the method offorming the RESURF layer in accordance with the third embodiment,wherein FIG. 9A shows the electric field reducing region immediatelyafter an ion implantation step and FIG. 9B shows the electric fieldreducing region after the ion-implanted impurities have been diffused bysubjecting the substrate to heat treatment. The method begins by formingan insulating film 63 a on the n-type region 21. A resist 63 b is thenformed on the insulating film 63 a. The resist 63 b includes resistportions R1 to R13 and openings defined by these resist portions. Theopenings have widths W1 to W13, which are equal. The widths of theresist R1 to R13 satisfy the following relation: width of R1<width ofR2<width of R3<width of R4<width of R5<width of R6<width of R7<width ofR8<width of R9<width of R10<width of R11<width of R12<width of R13. Ionsare then implanted using the resist 63 b as a mask, thereby forming aplurality of p-type regions in the principal surface 20 a of thesemiconductor substrate 20. FIG. 9A shows the p-type regions immediatelyafter they have been formed. The resist 63 b is then removed, and heattreatment is performed to diffuse the impurities in the plurality ofp-type regions and thereby simultaneously form the p-well layer 22 andthe RESURF layer 60, as shown in FIG. 9B.

In the semiconductor device of the third embodiment, since the impurityconcentration is high on the emitter side of the RESURF layer 60, thatside of the RESURF layer 60 is unlikely to be depleted (that is, thedepletion layer is unlikely to fully extend through the emitter side).As a result, the equipotential lines in the boundary, or the firstboundary region, between the p-well layer 22 and the RESURF layer 60 arerelatively widely spaced, making it possible to reduce the electricfield intensity at the first boundary region. On the other hand, sincethe impurity concentration is low on the channel stopper side of theRESURF layer 60, the channel stopper side of the RESURF layer 60 islikely to be depleted, making it possible to reduce the electric fieldintensity on that side of the RESURF layer 60. In this way the electricfield intensity across the surface of the RESURF layer 60 (i.e., acrossthe principal surface 20 a) can be substantially equalized to increasethe breakdown voltage of the semiconductor device. Therefore, inaccordance with the configuration of the semiconductor device of thethird embodiment, it is possible to manufacture a reduced sizesemiconductor device having an adequately high breakdown voltage.Further, since the p-well layer 22 and the RESURF layer 60 can besimultaneously formed, the above-described advantages of the thirdembodiment can be achieved at low cost.

The semiconductor device of the third embodiment is characterized inthat the peaks of the electric field intensity across the surface of theRESURF layer 60 and the adjoining surfaces are reduced by adjusting theimpurity concentration in the RESURF layer 60 so that the impurityconcentration is high on the emitter side of the RESURF layer 60 and lowon the channel stopper side. Therefore, the plurality of lower fieldplates 64 and the plurality of upper field plates 34 may be omitted fromthe semiconductor device.

Fourth Embodiment

FIG. 10 is a cross-sectional view of a semiconductor device inaccordance with a fourth embodiment of the present invention. Thesemiconductor device of the fourth embodiment has many features commonto the semiconductor device of the third embodiment. Therefore, thefollowing description of the semiconductor device of the fourthembodiment will be limited to the differences from the semiconductordevice of the third embodiment.

The RESURF layer 70 of the fourth embodiment is formed by first forminga plurality of p-type regions in the principal surface 20 a of thesemiconductor substrate 20 in such a manner that their surface areasgradually decrease as the regions are located further away from thep-well layer 22 toward the channel stopper 26, and then heat-treatingthe substrate. Specifically, the RESURF layer 70 includes a first region70 a in contact with the p-well layer, a second region 70 b in contactwith the first region 70 a, and a third region 70 c in contact with thesecond region 70 b. Each of the first region 70 a, the second region 70b, and the third region 70 c includes a plurality of p-type layershaving equal surface areas.

However, the surface area of the p-type layers of the second region 70 bis smaller than that of the p-type layers of the first region 70 a.Further, the surface area of the p-type layers of the third region 70 cis smaller than that of the p-type layers of the second region 70 b. Thedepth of the p-type layers of the second region 70 b from the principalsurface 20 a is less than that of the p-type layers of the first region70 a. The depth of the p-type layers of the third region 70 c from theprincipal surface 20 a is less than that of the p-type layers of thesecond region 70 b. The p-type impurity concentrations of the p-typelayers of the first region 70 a, the second region 70 b, and the thirdregion 70 c increase in the order named.

A method of forming the RESURF layer 70 will now be described. FIG. 11includes cross-sectional views showing the method of forming the RESURFlayer in accordance with the fourth embodiment, wherein

FIG. 11A shows the electric field reducing region immediately after anion implantation step and FIG. 11B shows the electric field reducingregion after the ion-implanted impurities have been diffused bysubjecting the substrate to heat treatment. The method begins by formingan insulating film 73 a on the n-type region 21. A resist 73 b is thenformed on the insulating film 73 a. The resist 73 b has openings havingwidths W1 to W13. The widths W1 to W4 are equal, the widths W5 to W8 areequal, and the widths W9 to W13 are equal, where W1>W5>W9. Ions are thenimplanted using the resist 73 b as a mask. Then, the resist 73 b isremoved, and heat treatment is performed to simultaneously form thep-well layer 22 and the RESURF layer 70.

The semiconductor device of the fourth embodiment has the sameadvantages as the semiconductor device of the third embodiment. Thesemiconductor device of the fourth embodiment differs from that of thethird embodiment in that the impurity concentration gradient in theRESURF layer is produced by use of a resist having openings of differentwidths. It should be noted that the plurality of lower field plates 64and the plurality of upper field plates 34 may be omitted from thesemiconductor device of the fourth embodiment, as is the case with thesemiconductor device of the third embodiment.

Fifth Embodiment

FIG. 12 is a cross-sectional view of a semiconductor device inaccordance with a fifth embodiment of the present invention. Thesemiconductor device of the fifth embodiment has many features common tothe semiconductor device of the third embodiment. Therefore, thefollowing description of the semiconductor device of the fifthembodiment will be limited to the differences from the semiconductordevice of the third embodiment.

The RESURF layer 80 of the fifth embodiment includes a first region 80a, a second region 80 b, and a third region 80 c. The p-type impurityconcentrations of the first region 80 a, the second region 80 b, and thethird region 80 c increase in the order named. A method of forming theRESURF layer 80 is to perform impurity introduction and heat treatmentfor forming the first region 80 a, then those for forming the secondregion 80 b, and then those for forming the third region 80 c. However,these impurity introduction steps are not limited to this particularorder. Further, heat treatment may be performed only after thecompletion of all the impurity introduction steps, instead of performingheat treatment after the completion of each impurity introduction step.

The semiconductor device of the fifth embodiment has the same advantagesas the semiconductor devices of the third and fourth embodiments.Furthermore, the fifth embodiment allows the RESURF layer to be formedwhile accurately controlling its impurity concentration, as comparedwith the third and fourth embodiments, making it possible to manufacturesemiconductor devices which vary little in characteristics. Although inthe third, fourth, and fifth embodiments the RESURF layer has animpurity concentration gradient in the lateral direction (parallel tothe principal surface 20 a), the present invention is not limited tothis particular impurity concentration profile of the RESURF layer. Theonly requirement for the impurity concentration of the RESURF layer isthat the p-type impurity concentration of the RESURF layer graduallydecreases away from the p-well layer toward the channel stopper layer.

Sixth Embodiment

FIG. 13 is a cross-sectional view of a semiconductor device inaccordance with a sixth embodiment of the present invention. In thesemiconductor device of the sixth embodiment, the structure on thesemiconductor substrate 20 is identical to that in the semiconductordevice of the first embodiment, and the RESURF layer is identical tothat of the semiconductor device of the third embodiment.

The semiconductor device of the sixth embodiment has an increasedbreakdown voltage. It should be noted that the electric field reducingregion has breakdown voltage enhancing effect as long as the RESURFlayer 60 is formed to have a p-type impurity concentration whichgradually decreases away from the p-well layer 22 toward the channelstopper 26. FIGS. 14 and 15 show variations of the semiconductor deviceof the sixth embodiment which have a RESURF layer different from that ofthe sixth embodiment. Specifically, FIG. 14 is a cross-sectional view ofa semiconductor device having the RESURF layer 70 shown in FIG. 10described in connection with the fourth embodiment. FIG. 15 is across-sectional view of a semiconductor device having the RESURF layer80 shown in FIG. 12 described in connection with the fifth embodiment.

Seventh Embodiment

FIG. 16 is a cross-sectional view of a semiconductor device inaccordance with a seventh embodiment of the present invention. Thesemiconductor device of the seventh embodiment has many features commonto the semiconductor device of the sixth embodiment shown in FIG. 13.Therefore, the following description of the semiconductor device of theseventh embodiment will be limited to the differences from thesemiconductor device of the sixth embodiment.

A concentration gradient reducing section 90 is formed in the portion ofthe p-well layer 22 adjacent the RESURF layer 60. The p-type impurityconcentration of the concentration gradient reducing section 90, whichconstitutes a portion of the p-well layer 22, is such that the p-typeimpurity concentration gradient between the p-well layer 22 and theRESURF layer 60 is reduced. The concentration gradient reducing section90 includes p-type regions 90 a, 90 b, and 90 c. It should be noted thatthe p-well layer 22, which includes the concentration gradient reducingsection 90, and the RESURF layer 60 are formed in the same process.

In the semiconductor device of the seventh embodiment, the concentrationgradient reducing section 90 serves to reduce the impurity concentrationgradient in the lateral direction (parallel to the principal surface 20a). Since the p-well layer 22 includes the concentration gradientreducing section 90, the curved surface of the p-well layer 22 has arelatively large curvature radius (as compared with the embodimentsdescribed above), making it possible to reduce the electric field atthat surface and increase the breakdown voltage of the semiconductordevice. It should be noted that the impurity concentration in someregion of the surface of the concentration gradient reducing section 90may have a suitable gradient (e.g., a Gaussian gradient) so as toproduce the effect described above.

The semiconductor device of the seventh embodiment is characterized bybeing provided with the concentration gradient reducing section 90 forreducing the electric field at the curved surface of the p-well layer22. A variety of alterations can be made to this semiconductor device aslong as this feature is retained. For example, FIGS. 17 and 18 showsemiconductor devices which are similar to that of the seventhembodiment, but have a different RESURF layer. Specifically, FIG. 17 isa cross-sectional view of a semiconductor device having the RESURF layer70 shown in FIG. 10 described in connection with the fourth embodiment.FIG. 18 is a cross-sectional view of a semiconductor device having theRESURF layer 80 shown in FIG. 12 described in connection with the fifthembodiment.

Eighth Embodiment

FIG. 19 is a cross-sectional view of a semiconductor device inaccordance with an eighth embodiment of the present invention. Thesemiconductor device of the eighth embodiment has many features commonto the semiconductor device of the second embodiment shown in FIG. 6.Therefore, the following description of the semiconductor device of theeighth embodiment will be limited to the differences from thesemiconductor device of the second embodiment.

A concentration gradient reducing section 92 is formed in the portion ofthe p-well layer 22 adjacent the RESURF layer 52. The p-type impurityconcentration of the concentration gradient reducing section 92, whichconstitutes a portion of the p-well layer 22, is such that the p-typeimpurity concentration gradient between the p-well layer 22 and theRESURF layer 52 is reduced. The concentration gradient educing section92 includes p-type regions 92 a, 92 b, 92 c, and 92 d. An insulatingfilm 100 is formed on the concentration gradient reducing section 92. Agate wire 104 is formed on the insulating film 100 directly above thep-type regions 92 a, 92 b, and 92 c, which are on the opposite side ofthe concentration gradient reducing section 92 from the RESURF layer 52.The gate wire 104 includes polysilicon 104 a and aluminum 104 b.

Further, an emitter grounding electrode 106 is formed directly above thep-type region 92 d, which is on the side of the concentration gradientreducing section 92 adjacent the RESURF layer 52. The emitter groundingelectrode 106 is connected to the emitter electrode 12 so that they areat the same potential. The emitter grounding electrode 106 is in contactwith the concentration gradient reducing section 92 of the p-well layer22 and extends over the insulating film 30 directly above the firstboundary region 23. The plurality of lower field plates 32 are formed insuch a manner that they do not lie directly above the first boundaryregion 23. Since, as described above, the emitter grounding electrode106 is formed on the insulating film 30 directly above the firstboundary region 23, the electric field intensity at the first boundaryregion 23 can be reduced in the manner described above in connectionwith the first embodiment.

It should be noted that in order to stabilize the potential of a gatewire, it is common to form a p-well layer or an extension region thereofunder the gate wire. The portion or extension of a p-well layer formedto stabilize the potential of a gate wire is hereinafter referred to asa “p-well layer extension portion.” A p-well layer extension portionformed in a semiconductor device takes some space, which has preventedminiaturization of semiconductor devices. In the semiconductor device ofthe eighth embodiment, however, the gate wire 104 is formed directlyabove the concentration gradient reducing section 92, eliminating theneed to form a p-well layer extension portion. This makes it possible tomanufacture semiconductor devices of reduced size.

Furthermore, since the lateral electric field (electric field parallelto the principal surface 20 a) is reduced by the RESURF layer, there isno lateral potential difference across the concentration gradientreducing section 92, making it possible to stabilize the potential ofthe gate wire 104. Thus, the concentration gradient reducing section 92of the eighth embodiment functions as a p-well layer extension portion,as well as serving to reduce the electric field at the curved surface ofthe p-well layer 22, as described above.

FIG. 20 is a cross-sectional view of a variation of the semiconductordevice of the eighth embodiment. A gate wire 105 is formed on aninsulating film 110 on a concentration gradient reducing section 94which includes p-type regions 94 a and 94 b. The gate wire 105 includespolysilicon 105 a and aluminum 105 b. The aluminum 105 b extends overthe insulating film 110 directly above the first boundary region 23.This semiconductor device is not provided with an emitter groundingelectrode.

This semiconductor device has the same advantages as the semiconductordevice of the eighth embodiment. Further, the gate wire 105, which isdisposed directly above the first boundary region, provides an electricfield intensity reducing effect equivalent to that of an emittergrounding electrode formed directly above the first boundary region.That is, the construction of the semiconductor device, which is notprovided with an emitter grounding electrode, results in a reduced sizeas compared with the construction of the eighth embodiment.

Features of the semiconductor devices of different embodiments describedabove may be combined where appropriate to provide a semiconductordevice having a reduced size and increased breakdown voltage.

As described above, all of the features of the present invention relateto the electric field reducing region; that is, the present invention isnot limited to any particular construction of the device forming region.Therefore, the present invention can be applied to any semiconductordevice which has an electric field reducing region formed therein formaintaining the breakdown voltage of the device. For example, thepresent invention may be applied to, in addition to IGBTs, MOSFETs,bipolar transistors, and diodes. Further, although the embodiments ofthe present invention have been described by specifying the conductivitytypes of the components of the semiconductor device, it is to beunderstood that the present invention is not limited to these particularconductivity types, and the conductivity type of each component may beeither a first conductivity type or a second conductivity type.

Although in the embodiments of the invention described above thesemiconductor substrate is formed of Si, it is to be understood that itmay be formed of a wide bandgap semiconductor having a wider bandgapthan Si. Examples of wide bandgap semiconductors include siliconcarbide, gallium nitride-based materials, and diamond.

In accordance with the present invention, the electric field intensityacross the RESURF layer of a semiconductor device can be madesubstantially uniform, making it possible to manufacture a semiconductordevice of reduced size, yet having an adequately high breakdown voltage.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2011-283871,filed on Dec. 26, 2011 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, are incorporated herein by reference in its entirety.

1. A semiconductor device comprising: a semiconductor substrate having aprincipal surface; a first impurity region of a first conductivity typeformed in said semiconductor substrate; a RESURF layer of a secondconductivity type formed in said semiconductor substrate along saidprincipal surface; a well layer of said second conductivity type formedin said semiconductor substrate adjacent said RESURF layer and alongsaid principal surface; a channel stopper of said first conductivitytype formed in said semiconductor substrate adjacent said RESURF layerand along said principal surface, with said first impurity regioninterposed between said channel stopper and said RESURF layer; aninsulating film formed on said principal surface and continuouslycovering a top surface of a first boundary region and a top surface of asecond boundary region, said first boundary region including a boundarybetween said well layer and said RESURF layer, said second boundaryregion including a boundary between said RESURF layer and said firstimpurity region; a plurality of lower field plates formed in saidinsulating film in such a manner that said plurality of lower fieldplates do not lie directly above said first and second boundary regions;and a plurality of upper field plates formed on said insulating film insuch a manner that said plurality of upper field plates do not liedirectly above said first and second boundary regions.
 2. Thesemiconductor device according to claim 1, further comprising: anemitter electrode formed in contact with said well layer and extendingover said insulating film directly above said first boundary region; anda channel stopper electrode formed in contact with said channel stopperand extending over said insulating film directly above said secondboundary region.
 3. The semiconductor device according to claim 2,wherein a first capacitance formed between a first lower plate amongsaid lower plates and said emitter electrode and a second capacitanceformed between a second lower plate among said lower plates and saidchannel stopper electrode are greater than a third capacitance formedbetween one of said lower plates and one of said upper plates, saidfirst lower plate being the closest of said lower plates to said firstboundary region, said second lower plate being the closest of said lowerplates to said second boundary region.
 4. The semiconductor deviceaccording to claim 1, wherein said RESURF layer is formed of a pluralityof regions of said second conductivity type.
 5. The semiconductor deviceaccording to claim 1, wherein said RESURF layer is formed to have animpurity concentration of said second conductivity type which graduallydecreases away from said well layer toward said channel stopper.
 6. Thesemiconductor device according to claim 1, wherein a concentrationgradient reducing section is formed in the portion of said well layeradjacent said RESURF layer in such a manner that the impurityconcentration gradient of said second conductivity type between saidwell layer and said RESURF layer is reduced.
 7. A semiconductor devicecomprising: a semiconductor substrate having a principal surface; afirst impurity region of a first conductivity type formed in saidsemiconductor substrate; a well layer of a second conductivity typeformed in said semiconductor substrate along said principal surface; achannel stopper of said first conductivity type formed in saidsemiconductor substrate away from said well layer and along saidprincipal surface; and a RESURF layer formed in said semiconductorsubstrate between said well layer and said channel stopper and alongsaid principal surface, and having an impurity concentration of saidsecond conductivity type which gradually decreases away from said welllayer toward said channel stopper.
 8. The semiconductor device accordingto claim 7, wherein said RESURF layer is formed of a plurality ofregions of said second conductivity type which are closely spaced on thewell layer side of said semiconductor substrate and widely spaced on thechannel stopper side of said semiconductor substrate.
 9. Thesemiconductor device according to claim 7, wherein said RESURF layer isformed of a plurality of regions of said second conductivity type whichgradually decrease in area and in depth from said principal surface assaid plurality of regions are located further away from said well layertoward said channel stopper.
 10. A semiconductor device comprising: asemiconductor substrate having a principal surface; a first impurityregion of a first conductivity type formed in said semiconductorsubstrate; a RESURF layer of a second conductivity type formed in saidsemiconductor substrate and along said principal surface; a well layerof said second conductivity type formed in said semiconductor substrateadjacent said RESURF layer and along said principal surface; aconcentration gradient reducing section formed in the portion of saidwell layer adjacent said RESURF layer in such a manner that the impurityconcentration gradient of said second conductivity type between saidwell layer and said RESURF layer is reduced; and a gate wire formeddirectly above said concentration gradient reducing section.
 11. Thesemiconductor device according to claim 10, further comprising anemitter grounding electrode formed directly above said concentrationgradient reducing section and connected to an emitter electrode.
 12. Thesemiconductor device according to claim 1, wherein said semiconductorsubstrate is formed of a wide bandgap semiconductor.
 13. Thesemiconductor device according to claim 12, wherein said wide bandgapsemiconductor is silicon carbide, gallium nitride-based material, ordiamond.